Apparatus for and method of irrigating soil

ABSTRACT

In a apparatus for irrigating soil for cultivating plants, a water supply pipe is buried in soil, plural porous pipes being connected to in a conduit while disposed at proper positions of the conduit in the water supply pipe, fine pores being provided in a wall surface of the porous pipe, the fine pores being not provided in a wall surface of the conduit that is able to pass water; both ends of the water supply pipe are connected to a water tank at positions lower than the porous pipe; a water surface of the water tank is usually maintained at a level horizontally equal to or lower than the porous pipe; the water supply pipe is saturated with water from the water tank; water is intermittently supplied to the water tank; and a position of the water surface in the water tank is set slightly higher than positions of the porous pipes buried in the soil, and a water partial pressure of the water tank is set to a slightly positive pressure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Conventionally, there have been proposed many methods for irrigating soil in which plants are cultivated. One of the methods is a water-saving irrigation method proposed by B. E. Livingston in 1934. A principle of the water-saving irrigation method is a method for irrigating the soil by utilizing a water pressure difference. In the water-saving irrigation method, a porous pipe is buried in the soil in which the plants are to be cultivated, the porous pipe is saturated with water, the soil is irrigated when the soil has a low water partial pressure, and excess fluid is taken in (drawn into) the porous pipe when the soil has a high water partial pressure. One of the features of this water-saving irrigation method is that a small amount of water is used.

2. Description of Related Art

Japanese Laid-open (Kokai) Patent Applications Publications Nos. 9-9802 and 9-308396 propose techniques as examples of practical use of methods for irrigating soil by utilizing a water pressure difference. According to the techniques proposed therein, gases such as air dissolved in water in a water supply system (or a water transporting system) including the porous pipe and a conduit may come out of solution as bubbles due to changes in environment such as changes in temperature, and occasionally, the bubbles adhere to an inner surface of the porous pipe and block fine pores, which results in a problem in that liquid cannot be supplied to the soil even if a pressure difference exists between the inside and the outside of the porous pipe. The inventors earnestly studied the problem and proposed a method described in Japanese Patent No. 4173228. According to the method described in Japanese Patent No. 4173228, the remaining bubbles can be substantially completely removed by quickly passing water through the water transporting system at a water pressure of 2 to 3 kg/cm², generating a pressure difference between the inside and the outside of the porous pipe, and the liquid can be supplied to the soil when the porous pipe has high water pressure.

However, as a result of subsequent research, occasionally, no pressure difference is generated between the inside and the outside of the porous pipe even if the remaining bubbles can be substantially completely removed. As a result of study of the cause, the following knowledge was obtained. That is, it was found that soil dries and shrinks because of a rapid changes in the external environment such as rapid rise of outside air temperature, rapid vaporization of soil water by drying (low humidity) of air, and the rapid absorption of the soil water by plants at peaks of growth, and the soil that is in contact with the surface of the porous pipe separates from the surface of the porous pipe. In order to supply the irrigation water by utilizing the water pressure difference, it is necessary that the soil be in contact with the surface of the porous pipe, and it is necessary that the soil be in a state in which capillary action can be maintained. Therefore, when the soil that is in contact with the surface of the porous pipe peels off from the surface of the porous pipe, the soil cannot be irrigated by utilizing the water pressure difference.

SUMMARY OF THE INVENTION

The inventors completed the present invention after earnestly studying the problem existing in the conventional technique, that is, the technique of being able to stably irrigate the soil by utilizing the water pressure difference even if a rapid change of the external environment such as rapid rise of the outside air temperature, rapid vaporization of soil water by drying (low humidity) of the air, and rapid absorption of soil water by the plant at peak growth occurs.

In accordance with an aspect of the invention, a apparatus for irrigating soil for cultivating plants includes: a water supply pipe buried in soil, having multiple porous pipes and a conduit, the multiple porous pipes being connected to the conduit disposed at appropriate locations of a conduit to the water supply pipe, fine pores being provided in a wall surface of the porous pipe, fine pores not being provided in a wall surface of the conduit for transporting water; and a water tank to which both ends of the water supply pipe are connected to at positions lower than the porous pipe, a water surface of the water tank usually maintained at a level horizontally equal to or lower than the porous pipe, the water supply pipe saturated with water from the water tank, and (b), a fluid in the porous pipe leaching to the soil when a water partial pressure (a) of the porous pipe>a water partial pressure (b) in the soil that is in contact with a surface of the porous pipe, and the fluid contained in the soil is drawn into the porous pipe when (a)<(b), water is intermittently supplied to the water tank, and a position of the water surface in the water tank is set slightly higher than positions of the porous pipes buried in the soil, and a water partial pressure of the water tank is set to a slightly positive pressure.

In accordance with an another aspect of the invention, a method for irrigating soil for cultivating plants includes: providing a water tank for holding water, a water supply pipe for transporting water, and a porous pipe with fine holes that allow water to pass therethrough; connecting the water tank, the water supply pipe, and the porous pipe; burying the water supply pipe so that ends of the water supply pipe are lower than the porous pipe; burying the porous pipe; supplying water to the water tank so that the level of the top of the water does not exceed the level of the porous pipe, in which water passes from the inside of the porous pipe to the soil when the water partial pressure inside the porous pipe is higher than the water partial pressure in the soil.

In particular, the invention has the following prominent effects, and the invention has a great deal of potential in industry:

1. According to the method of the invention for irrigating the soil by utilizing the water pressure difference, the soil can be stably and continuously irrigated even if the external environment is rapidly changed by the rapid rise of the outside air temperature, the rapid vaporization of the soil water due to the drying (low humidity) of air, and the rapid absorption of the soil water due to plants being at peak growth; and

2. According to the method of the invention for irrigating the soil by utilizing the water pressure difference, it is not necessary to continuously monitor the water level of the water tank, the irrigation operation is simple, and risk of excess irrigation is eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating an embodiment of the invention; and

FIG. 2 is a schematic view illustrating another embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Exemplary embodiments of the invention will be described in detail. The invention relates to an apparatus and method for irrigating plants cultivated in the soil. In the irrigation method, a water supply pipe is buried in soil, plural porous pipes are connected to a conduit (hereinafter sometimes simply referred to as conduit (A)) while disposed at proper positions of the conduit (A) in the water supply pipe, fine pores are provided in a wall surface of the porous pipe (hereinafter sometimes simply referred to as conduit (B)), the fine pores are not provided in a wall surface of the conduit (A), the water is caused to leach from the fine pores of the conduit (B) to the outside by saturating the water supply pipe with the water, and the leaching water is used to grow and cultivate plants.

There is no particular limitation to the soil used in the method of the invention as long as the soil can usually retain a fluid among particles to cultivate plants. Natural soil or soil artificially obtained by crushing and mixing, or a mixture of an artificial soil and a natural soil may be used. All kinds of soil based on the classifications of “Japanese Unified Soil Classification” can be cited as the soil of the invention. That is, the soil is classified into coarse particles (gravel particles and sand particles), fine particles, and highly organic particles (peat and black mud) based on an earth material, and any particle may be used in the soil of the invention. The particles having large particle diameter are not only used, but are preferably mixed with sand particles or fine particles (silt, cohesive soil, organic soil, and volcanic cohesive soil). Soil having many gravel particles can be improved so as to be soil suitable for growing plants by a method for dressing the fine particles or the highly organic particles. Soil that is not inclined but is flat is suitable for a soil to which the method of the invention can be applied. Examples of the soil include fields and greenhouses having various areas and soil for potted plants or planters having smaller area.

The conduit (A) in which the fine pores are not provided in the wall surface constituting the water supply pipe constitutes a portion in which the water supply pipe is not in buried in the soil, and it is connected to the conduit (B) at constant or freely selected intervals in a portion in which the conduit (A) is buried in the soil. A conduit (tube) that is made of a thermoplastic resin and produced by extrusion is suitable for the conduit (A). Examples of the resin include polyethylene, polypropylene, polyvinyl chloride, polybutene, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polycarbonate, and polyamide. Among others, polyvinyl chloride, polyethylene, and polypropylene are suitably used as the resin. There is no limitation on inner diameter, wall thickness, and length of the conduit. However, water resistance becomes large in supplying water when the inner diameter of the conduit is excessively small, and a large amount of water is required to flush out bubbled generated in the water supply pipe or a bubble mixed in from the outside of the water supply pipe when the inner diameter is excessively large. The inner diameter and wall thickness of the conduit depend on soil environment (for example, a region, a field or a greenhouse, kind of soil, and kind of cultivated plant) where the water supply pipe is buried. Usually, the inner diameter ranges from 1 to 50 mm, and it is preferably from 3 to 30 mm. The wall thickness ranges from 0.5 to 20 mm, and it is preferably from 1 to 10 mm in proportion to the inner diameter.

The fine pores provided in the wall surface of the conduit (B) have a function of passing the water or moisture vapor from the inside of the porous pipe to the outside of the porous pipe or from the outside of the porous pipe to the inside of the porous pipe. A porous conduit made of ceramics, concrete, porous glass, a metallic sintered body, and a thermoplastic resin such as polyethylene, polypropylene, and rubber can be used. Additionally, a conduit in which the metallic sintered body or a material being able to be used as a filter material is cylindrically formed can be used as the porous pipe. Among others, a conduit in which pot clay having a large content of quartz is formed and fired is suitably used.

The hole diameter of the fine pore of the conduit (B) ranges from 0.01 to 200 μm. When the hole diameter is less than 0.01 μm, not only is clogging easily generated during use, but also the water resistance increases in supplying the water. When the hole diameter is greater than 200 μm, for the water level of the water tank that is much lower than the horizontal position of the porous conduit, because air contained in the soil is drawn in to cause the water in the water supply pipe to drop easily in the water tank, the state in which the water supply pipe is saturated with water cannot be maintained. More preferably, the hole diameter ranges from 0.1 to 50 μm. The inner diameter and wall thickness of the conduit (B) are preferably set identical to those of the conduit (A), but it is not always necessary that they be identical to those of the conduit (A). The fine pores are suitably processed by a hydrophilic material because water or moisture vapor is easily passed therethrough. When the conduit (B) is excessively short, operation becomes difficult in connecting the conduit (B) to the conduit (A) to form the water supply pipe. When the conduit (B) is excessively large, cost increases, which is undesirable. Usually, the length of the conduit (B) ranges from 30 to 200 mm.

As to a procedure of preparing the water supply pipe and burying the water supply pipe in the soil, the kind of the material, the diameter, the inner diameter, and the wall thickness of the conduit (B) are selected according to the kind of soil in which the water supply pipe is buried and the kind of plant cultivated in the soil. At the same time, the kind of material, the diameter, the inner diameter, the wall thickness, and the hole diameter of the conduit (A) are selected. Then, the portion in which the water supply pipe is not buried is formed only by the conduit (A), and the conduits (B) are connected to the conduit (A) at constant or freely selected intervals in the portion in which the water supply pipe is buried, which allows the water to be supplied. When the plural conduits (B) are connected to the conduit (A), the number of conduits (B) and the intervals between the conduits (B) can be appropriately selected according to the soil environment (for example, the region, the field or greenhouse, the kind of soil, and the kind of cultivated plant). Preferably, a connecting tool is used in a joint between the conduit (A) and the conduit (B). Not only the connecting tool connects the conduit (A) and the conduit (B), but also the connecting tool controls the conduit (A) and the conduit (B) when the conduit (A) differs from the conduit (B) in diameter, inner diameter, and wall thickness. The connecting tool may be made of a thermoplastic resin or metal.

The conduit (A) and the conduits (B) are connected to form the water supply pipe, and the water supply pipe is buried in the soil. In addition to the method for burying the previously assembled long water supply pipe in the soil, a method for sequentially burying the conduit (A) and the conduits (B) while controlling the length of the conduit (A) and the number and the length of the conduits (B) according to situation (flat or ridged, or the width and length of the ridge when the entire area is ridged) of the soil may be adopted. In burying the water supply pipe in the soil, a depth is appropriately selected according to the soil environment (for example, the region, the field or greenhouse, the kind of soil, and the kind of cultivated plant). Usually the depth from the soil surface ranges from 50 to 200 mm.

The water tank has a function of retaining the water that irrigates the soil. Both ends of the water supply pipe are connected to the water tank at a position below the position of the porous pipe, and the water supply pipe is connected to the water tank while saturated with the water (see FIGS. 1 and 2). The water surface of the water tank is maintained at a level horizontally equal to or slightly lower than that of the porous pipe. Even if the water surface of the water tank is lower than the horizontal position of the porous pipe, the water with which the water supply pipe is saturated can be maintained by a siphon effect.

In the irrigation method of the invention in which a water pressure difference is used, a fluid in the porous pipe is caused to leach onto the soil side when a water partial pressure (a) of the porous pipe>a water partial pressure (b) a water partial pressure included in the soil that is in contact with a surface of the porous pipe, and the fluid contained in the soil is taken in (drawn into) the porous pipe when (a)<(b). When the water partial pressures have the former relationship, even if the water surface of the water tank is lower than the porous pipe, the water leaches to the soil by capillary action and is continuously supplied to a root of a plant in the state in which the water supply pipe is saturated with the water to cover an outside surface of the conduit (B) with the soil containing the moisture. When the water partial pressures have the latter relationship, the excess fluid contained in the soil can be taken in (drawn into) the porous pipe and delivered to the water tank in the state in which the water supply pipe is saturated with the water to cover the outside surface of the conduit (B) with the soil containing the moisture while the water surface of the water tank is lower than the porous pipe.

In the irrigation method of the invention, as described above, the fluid leaches to the soil side by capillary action and is supplied to the roots of plants, in the state in which the water supply pipe is saturated with the water to cover the outside surface of the conduit (B) with the soil containing the moisture. Usually, the water surface of the water tank is maintained at a level horizontally equal to or lower than that of the porous pipe. At this position, because the lower position depends on the area of the irrigated soil, the kind of plant grown and cultivated in the soil, a growth period (germinating period or growing period), and a size of the water tank, the lower position can appropriately be changed depending on the situation. Usually the lower position ranges from 10 to 300 mm. Any kind of water such as rainwater, river water, groundwater, and tap water can be used as long as the water is suitable for plant growth. As the need arises, nutrients such as fertilizer necessary to grow the plant can be dissolved in water. There is no particular limitation to the plants that can be grown and cultivated in the soil. Examples of the plants include various vegetables, flowers, fruits, and trees.

When the fluid in the soil with which the outside surface of the conduit (B) is covered runs out, the soil dries and shrinks, and the soil separates from the outside surface of the conduit (B). The capillary action is not generated after the soil separates off, and the water cannot be supplied to the soil. In the irrigation method of the invention, in order to prevent the soil from peeling off from the outside surface of the conduit (B), immediately before the soil peels off from the outside surface of the conduit (B) (for example, in about half a day to about two days), the position of the water surface in the water tank is intermittently set slightly higher than the positions (horizontal position) of the conduits (B) buried in the soil, and the water is actively caused to leach from the surface of the conduit (B) onto the soil side. Therefore, the water is supplied to the just drying soil again, and the soil becomes moist, so that the soil can be prevented from peeling off from the outside surface of the conduit (B) while the capillary action can be maintained. The slightly higher position depends on the soil environment (for example, the region, the field or greenhouse, the kind of the soil, and the kind of cultivated plant), the area of the irrigated soil, the kind of the plant grown and cultivated in the soil, the growth period (germinating period or growing period), and the size of the water tank, the slightly higher position can appropriately be changed depending on the situation. Usually, for example, the slightly higher position ranges from 10 to 50 mm. The position of the water surface in the water tank is set slightly higher than the positions of the conduits (B), which allows the generation of slightly positive pressure of about 0.003 to about 0.005 kg/cm².

Examples of a specific method for intermittently supplying the water to the water tank to set the position of the water surface in the water tank slightly higher than the position of the porous pipe buried in the soil include (1) a method for supplying the water by sensing the position of the water surface in the water tank with a water level detecting device, (2) a method for supplying the water at constant intervals using a timer, and (3) a method for supplying the water by sensing a soil water concentration of the outside surface of the porous pipe with a sensing device.

In the method (1), the water level detecting device is disposed in the water tank. Preferably the water level detecting device has a mechanism in which the water level is detected with a water level sensor and a signal is transmitted to a switch of a water supplying electromagnetic valve to open and close the electromagnetic valve when the water level reaches a high or low setting position. The water level sensor can be formed by a pair of sensors that detect the low and high water levels (described later; see FIG. 1). The water level sensor is set such that a water supply valve is opened to supply the water to the water tank when the water level of the water tank becomes lower than the setting position, and such that the water supply valve is closed when the water level of the water tank reaches the setting position. A product can properly be selected from various commercially available float switches and used as the water level sensor. In an alternative of the method (1), the high water level and the low water level are detected by a float ball disposed in the water tank, and a ball shut-off valve is opened and closed to supply the water to the water tank.

In the method (2) in which the water is supplied at constant intervals using the timer, an automatic irrigation timer is provided, a set amount of water can be supplied at a set time of day for a set interval of time. The water supply time of day, the supply time per each time, and the supply amount depend on the region where the soil exists, a property of the soil, the area of the irrigated soil, the kind of the plant grown and cultivated in the soil, the soil environment such as the growth period (germinating period or growing period), and the size of the water tank, so that the water supply time of day, the supply time per each time, and the supply amount can be changed depending on the situation based on experience and prior experiments. A product can properly be selected from various commercially available products and used as the automatic irrigation timer.

In the method (3), a water concentration sensor that detects the water concentration of covering soil is buried in the covering soil in the outside surface of the porous pipe buried in the soil, thereby sensing the soil water concentration. Preferably, when the water concentration becomes equal to or less than the set concentration, the signal is transmitted to the switch of the water supplying electromagnetic valve, and the electromagnetic valve is opened to supply the water for a constant interval of time. During operation of the switch, the water concentration and the water supply time can be determined by prior experiments and experience. A proper product can be selected from various commercially available water concentration sensors and used as the soil water concentration sensor.

As described above, the method of the invention can be applied to soil on the roof of a building, soil for a potted plant (including pot plant cultivated plants for greening a building wall surface) having a small area, and the soil of a planter for growing and cultivating plants, in addition to fields and greenhouses having various areas.

Embodiments of the invention will be described in detail with reference to the drawings. However, the invention is not limited to the embodiments. FIG. 1 is a schematic view illustrating a soil irrigating method according to an embodiment of the invention. In FIG. 1, the numeral 1 designates a water pipe, the numeral 2 designates a water tank, the numeral 3 designates plant cultivation soil, the numeral 4 designates a plant, the numerals 5 and 6 designate conduits, the numeral 7 designates a porous pipe, the numeral 8 designates water, the numeral 9 designates a horizontal position of the porous pipe, the numerals 10 and 11 designate water level sensors, the numeral 10′ designates a leading end position of the lower limit sensor, the numeral 11′ designates a leading end position of the upper limit sensor, the numeral 12 designates a switch of an electromagnetic valve, and the numeral 13 designates the electromagnetic valve. In FIG. 1, the porous pipe 7 in which the fine pores are provided in the wall surface is connected to a proper position of the conduit 5 to form the water supply pipe, and the porous pipe 7 and the conduit 5 are buried in the plant cultivation soil 4. The fine pores are not provided in the wall surface of the conduit 5, and the conduit 5 can pass the water. Both ends of the water supply pipe are connected to the water tank 2 by the conduits 5 and 6 at positions lower than the horizontal position 9 of the porous pipe 7. In the water tank 2, the water 8 is maintained at a level equal to or lower than the horizontal position 9 of the porous pipe 7, whereby the water supply pipe is saturated with the water 8 from the water tank 2. In order to saturate the water supply pipe with the water 8, the water may be supplied into the water supply pipe at a pressure of about 2 to 3 kg/cm² while air is forcedly removed.

Even if the water surface of the water tank 2 is lower than the horizontal position 9 of the porous pipe 7, as described above, the water 8 with which the water supply pipe is saturated remains in the water supply pipe by the siphon effect without dropping in the water tank 2, thereby maintaining the saturated state in the water supply pipe. When a water partial pressure (a) of the porous pipe 7>a water partial pressure (b) of the soil 4 that is in contact with the surface of the porous pipe 7, the fluid in the porous pipe 7 leaches from the fine pores onto the side of the soil 4, and the water can be supplied to the plant grown and cultivated in the soil 4. When (a)<(b), the fluid contained in the soil 4 is taken in (drawn into) the porous pipe 7 to be able to control the fluid and moisture of the soil 4.

When the water surface of the water tank 2 is lowered, in the embodiment illustrated in FIG. 1, the two water level sensors 10 and 11 detect a level of the water in the water tank 2. A leading end of the lower limit sensor 10 detects the lower limit position 10′ of the water surface, and an on-signal is transmitted from the switch 12 to the electromagnetic valve 13 to open the electromagnetic valve 13, thereby supplying the water to the water tank 2. A leading end of the upper limit sensor 11 detects the upper limit position 11′ of the water surface, and an off-signal is transmitted from the switch 12 to the electromagnetic valve 13 to close the electromagnetic valve 13, thereby stopping the supply of the water to the water tank 2. Therefore, the surface of the water 8 in the water tank 2 can be controlled within a narrow range across the horizontal position 9 of the porous pipe 7. As described above, the lower limit position 10′ of the lower limit sensor and the upper limit position 11′ of the upper limit sensor can be determined by prior experiment and experience, and the lower limit position 10′ and the upper limit position 11′ are selected in the range of +50 mm to −300 mm across the horizontal position 9 of the porous pipe 7, depending on the size of the water tank 2. The water level sensor may be the float switch as described above.

FIG. 2 is a schematic view illustrating a soil irrigating method according to another embodiment of the invention. In FIG. 2, the numeral 21 designates a water pipe, the numeral 22 designates a water tank, the numeral 23 designates plant cultivation soil, the numeral 24 designates a plant, the numerals 25 and 26 designate conduits, the numeral 27 designates a porous pipe, the numeral 28 designates water, the numeral 30 designates a float ball, the numeral 31 designates a ball shut-off valve, the numeral 32 designates a ball shut-off valve support portion, and the numeral 33 designates an automatic irrigation timer. In FIG. 2, the porous pipe 27 in which the fine pores are provided in the wall surface is connected to a proper position of the conduit 25 to form the water supply pipe, and the porous pipe 27 and the conduit 25 are buried in the plant cultivation soil 23. The fine pores are not provided in the wall surface of the conduit 25, and the conduit 25 can transport the water. Both ends of the water supply pipe are connected to the water tank 2 by the conduits 25 and 26 at positions lower than the porous pipe 27. In the water tank 22, the water 28 is maintained at a level equal to or lower than the horizontal position of the porous pipe 27, whereby the water supply pipe is saturated with the water 28 from the water tank 22. In order to saturate the water supply pipe with the water 28, the water may be supplied into the water supply pipe at a pressure of about 2 to 3 kg/cm² while air is forcedly removed.

In the embodiment illustrated in FIG. 2, the water level of the water tank 22 is usually set so as to be always maintained constant, while the float ball 30 and the ball shut-off valve 31 operates with each other. However, because there is a risk of generating soil drying and soil shrinkage due to rapid change in the external environment (such as the outside air temperature and the humidity), preferably the automatic irrigation timer 33 is also provided to correspond to the rapid change of the external environment. The automatic irrigation timer 33 is connected to the water pipe 21, and a set amount of water can be supplied a set number of times at a set time of day for a set interval of time. As described above, the irrigation time of day, the number of irrigation times per day, the irrigation interval of time for each time of the irrigation, and the amount of irrigation, which are performed by the automatic irrigation timer 33, can be determined by the prior experiment and the experience.

It should be noted that the entire content of Japanese Patent Application No. 2009-205811, filed on Sep. 7, 2009, on which convention priority is claimed, is incorporated herein by reference.

It should also be understood that many modifications and variations of the described embodiments of the invention will occur to a person having ordinary skill in the art without departing from the spirit and scope of the present invention as claimed in the appended claims. 

1. An apparatus for irrigating soil for cultivating plants, comprising: a water supply pipe buried in soil, including multiple porous pipes and a conduit, the multiple porous pipes being connected to the conduit disposed at appropriate locations of the conduit to the water supply pipe, fine pores being provided in a wall surface of the porous pipe, the fine pores not being provided in a wall surface of the conduit for transporting water; and a water tank to which both ends of the water supply pipe are connected at positions lower than the porous pipe, a water surface of the water tank being usually maintained at a level horizontally equal to or lower than the porous pipe, the water supply pipe being saturated with water from the water tank, wherein a fluid in the porous pipe leaches to the soil side when a water partial pressure (a) of the porous pipe>a water partial pressure (b) in the soil that is in contact with a surface of the porous pipe, and the fluid contained in the soil is taken into (drawn into) the porous pipe when (a)<(b), wherein water is intermittently supplied to the water tank, and a position of the water surface in the water tank is set slightly higher than positions of the porous pipes buried in the soil, and a water partial pressure of the water tank is set to a slightly positive pressure.
 2. The apparatus for irrigating soil for cultivating plants according to claim 1, wherein the conduit is made of a thermoplastic resin, an inner diameter of the conduit ranges from 1 to 50 mm, and a wall thickness of the conduit ranges from 0.5 to 20 mm.
 3. The apparatus for irrigating soil for cultivating plants according to claim 1, wherein a hole diameter of the porous pipe ranges from 0.01 to 200 μm, an inner diameter of the porous pipe ranges from 5 to 50 mm, and a wall thickness of the porous pipe ranges from 0.5 to 30 mm.
 4. The apparatus for irrigating soil for cultivating plants according to claim 1, wherein the position of the water surface in the water tank is detected by a water level detecting device.
 5. The apparatus for irrigating soil for cultivating plants according to claim 4, wherein the water level detecting device has a mechanism in which a water level is detected by a water level sensor and an instruction is provided to a switch of a water supplying electromagnetic valve to open and close the water supplying electromagnetic valve.
 6. The apparatus for irrigating soil for cultivating plants according to claim 1, wherein the automatic irrigating timer is connected to the water pipe.
 7. A method for irrigating soil for cultivating plants, comprising: providing a water tank for holding water, a water supply pipe for transporting water, and a porous pipe with fine holes that allow water to pass therethrough; connecting the water tank, the water supply pipe, and the porous pipe; burying the water supply pipe so that ends of the water supply pipe are lower than the porous pipe; burying the porous pipe; supplying water to the water tank so that the level of the top of the water does not exceed the level of the porous pipe; wherein water passes from the inside of the porous pipe to the soil when the water partial pressure inside the porous pipe is higher than the water partial pressure in the soil. 